But according to Cate Blanchett, the secret to a successful marriage is a shared email account. The Oscar-nominated Australian actor this week told the Telegraph that the key to her 16-year relationship with writer-director Andrew Upton and their “well-oiled” household was her ability to synchronise their lives through a single email address. This counsel to seek romantic longevity in the inbox rather than the bedroom may come as a disappointment to some. However, it reflects the reality that increasingly, couples are merging their virtual as well as their private and public lives.

For some, establishing shared email or other social media accounts is a new marker of commitment, like a joint bank account or shared house-key. In instances of cyber-aversion, as is reportedly Upton’s case, there is an appealing pragmatism to such arrangements. Someone wading through my LinkedIn updates and Amazon receipts seems a strong grounds for pledging my unconditional love.

However, volunteering to share passwords, points to the more obvious question of trust and transparency. It allows scrutiny of all facets of our social as well as bureaucratic selves – not merely the fact that you still subscribe to Jennifer Anniston’s Buzzfeed, but if and when you last logged onto OKCupid or the unpalatable truth of what you really think about your colleagues/in-laws/acquaintances.

Opening up one’s email account is the new going-to-the-loo-with-the-door-open relationship frontier. Either you conduct your affairs in an extremely sanitary manner, or you are prepared to be caught doing things that are undignified, unpleasant and probably taboo.

The revelations found in most our current-day inboxes means that for many relationships they are the last bastion of trust. While recent studies document that one in five men keeps a secret email address for their partner, other entrepreneurs are making a lucrative trade from hacking the accounts of suspect spouses and lovers. So too, some Christian groups are now encouraging establishing joint accounts as a symbol of openness and fidelity between couples – a bulwark against the many snares and trespasses of the web. Equally, psychologists have endorsed such signs of openness and honesty, but warded against resorting the practice for reasons of mere deterrence or insecurity.

So what statements are our inboxes making about our relationships? As we cleave unto one another to become one flesh, must we also cleave unto Gmail? I am a serial (probably imprudent) sharer of toothbrushes, underpants and pin-numbers and have unthinkingly disclosed email passwords to lovers and loved-ones. But there is something decidedly unsettling in the thought of being exposed and accountable, not only for my final eBay bid on those novelty bookends, but also for all the many and varied identities I assume in other relationships – familial, social or professional.

Just as there is nothing less sexy than someone else’s Paypal receipts, it seems more desirable as well as more interesting to retain an element of mystique, or at least respectful autonomy, in our technologically-mediated lives. Social media has become yet another hallmark of identity in an individualism-obsessed culture. But retaining symbolic boundaries between our romantic relationships and broader identities does not mean retaining secrets. Rather, there seems to be value in making a healthy, trusting acknowledgement that who we are in our romantic partnerships does not and cannot reflect the whole of who we are in our relations with the world.

The credit card or house-key, like the email password, might be shared for practical reasons, but it is the assumption that you won’t come home to find it being used to cut cocaine on your kitchen bench (or otherwise abused), even in less-than-rosy relationship times, that is the real marker of trust.

According to Balzac, a man ought not marry without having studied anatomy and dissected at least one woman. Indeed, there may be merit in such studious engagement with one’s life partner, but it’s probably best not to extend the dissection to our inboxes – it may be a far sight messier.